In recent years, many portable electronic apparatuses such as an integral VTR/video camera unit, portable telephone, portable computer, etc. have been proposed, and they show a tendency to be more compact for their improved portability. Many developments and studies have been made to provide a thinner or bendable battery, more specifically, a secondary battery, or a lithium ion battery among others, for use as a portable power source in such a more compact portable electronic apparatus.
To attain such a thinner or bendable battery structure, active studies have been made concerning a solidified electrolyte for use in the battery. Especially, a gel electrolyte containing a plasticizer and a polymeric solid electrolyte made from a high molecular material having lithium salt dissolved therein are attracting much attention from many fields of industry.
As the high molecular materials usable to produce a high molecular solid electrolyte, a silicone gel, acryl gel, acrylonitrile, polyphosphazen-modified polymer, polyethylene oxide, polypropylene oxide, their composite polymer, cross-linked polymer, modified polymer, etc. have been reported. In the conventional secondary battery using a solid electrolyte made from one of these high molecular materials, however, since the electrolyte film has no sufficient film strength and adhesion to the battery electrodes, there occurs a nonuniformity between the charge and discharge currents, and a lithium dendrite easily takes place. Thus, the conventional secondary battery has a short charge and discharge cycle life (number of charge and discharge cycles), namely, it is critically disadvantageous in that it cannot meet the requirement “stable usability for a longer term” being one of the basic and important requirements for production of a commercial article.
Further, for a higher film strength of a solid electrolyte, it has been proposed to cross-link a trifunctional polyethylene glycol and diisocyanate derivative by reaction between them (as disclosed in the Japanese Unexamined Patent Publication No. 62-48716) or to cross-link polyethylene glycol diacrylate by polymerization (as disclosed in the Japanese Unexamined Patent Publication No. 62-285954). Because an unreacted substance or a solvent used for the reaction remains, the electrolyte has no sufficient adhesion to the battery electrodes. Moreover, the indispensable process of drying removal causes the productivity to be low. These methods are required for a further improvement.
As mentioned above, the high molecular solid or gel electrolyte has excellent characteristics not found with the liquid electrolytes, but when it is used in a battery, it can hardly be put in ideal contact with the battery electrodes. This is because the solid or gel electrolyte will not flow as the liquid electrolyte.
The contact of the high molecular solid or gel electrolyte with the battery electrodes has a large influence on the battery performance. Namely, if the contact between them is poor, the contact resistance between the high molecular solid or gel electrolyte and the battery electrodes is large so that the internal resistance of the battery is large. Furthermore, there cannot be an ideal ion movement between the high molecular solid or gel electrolyte and the electrodes, and so the battery capacity is also low. If such a battery is used for a long term, there occurs a nonuniformity between the charge and discharge currents and a lithium dendrite is likely to take place.
Therefore, in a battery using a high molecular solid or gel electrolyte, it is extremely important to adhere the high molecular solid or gel electrolyte to active material layers of electrodes of the battery with a sufficient adhesive strength.
To implement the above, it has been proposed as in the Japanese Unexamined Patent Publication No. 2-40867 to use a positive electrode composite in which a high molecular solid electrolyte is added to a positive active material layer of the positive electrode. In the battery disclosed in the Japanese Unexamined Patent Publication, a part of the high molecular solid electrolyte is mixed in the positive active material layer to improve the electrical contact between the high molecular solid electrolyte and positive-electrode active material layer.
However, in case the method disclosed in the Japanese Unexamined Patent Publication No. 2-40867 is adopted, the positive-electrode composite to which the high molecular solid electrolyte is added must be used to produce a positive plate and the high molecular solid electrolyte should be laminated on the positive plate. No ideal contact can be attained between the positive plate and solid electrolyte. More specifically, if a solid electrolyte having an irregular surface is laminated on an electrode layer, no good adhesion between them can be ensured and the internal resistance will be increased, with a result that the load characteristic becomes worse. Also, a positive or negative electrode composite in which a high molecular solid or gel electrolyte is added cannot easily be pressed to a sufficient extent because of the elasticity of the high molecular solid or gel electrolyte, and the grain spacing inside the composite is large, with a result that the internal resistance is increased. Also in this case, the load characteristic becomes worse. Furthermore, to prevent an electrolyte salt contained in the high molecular solid or gel electrolyte from being dissolved, the positive or negative electrode should be produced at a low humidity, their quality cannot easily be controlled, and the manufacturing costs are large.